(A) Is she any good?
(B) Does it matter?
Is She Any Good?
Kinda. There are a lot of people racing in Indy Car and the NASCAR Nationwide series - the two places she races today. She races a full Indy Car schedule, so lets look there first.
Danica's Indy Car Stats. She had a great year in 2005 and has come down from that a bit in recent years, but this year is on pace to match 2010, when she ranked 10th overall. She is currently 11th. In perspective, there are 43 drivers, but only about 20 of them run the full schedule, so she's in the middle of the pack. That puts her above everyone you've never heard of and below everyone you have (or might have...Dario Franchiti, Scott Dixon, Sam Hornish, etc.). So she ain't bad, but she's bringing up the middle of the pack.
The story is the same in Nationwide. She's 27th overall, but she has run just a 1/3 of the races - 6 in total. In points, she well outclasses other drivers with 6 races, and, in fact, drivers with 7,8, and 9 races, except Sam Hornish - her Indy Car rival. If we 'annualized' her points (i.e. assumed she'd have the same proportion of points if she'd run 21 races), she'd have 623 points, putting her in 7th place. That's not bad, but I should note that it is again just over 1/2 way up the list of 16 drivers that have run a full Nationwide schedule.
So Does It Matter That Danica Be Good?
This is fuzzier. NASCAR wants a good woman driver. Second, being a popular NASCAR driver is arguably just as important as being good. Dale Earnhardt Jr. has had several poor seasons. Tony Stewart is not at the top of the points standings. Carl Edwards has only again become a contender this year. Jeff Gordon, again spotty for a few years. All are more popular than Jimmie Johnson, who has won the points standings in NASCAR 5 years running. So personality and marketability, coupled with a respectable race record might be enough.
If I may, I would like to point out that great women drivers exist already. Ashley Force Hood - daughter of John Force, placed second overall in the NHRA Funny Car standings in 2009 on her father's team. No girls' division, no minor league ball. This was the big show and she killed it week after week on the line and the shifter. Then again, that was a small media market, so she hasn't gotten her due.
So I think Danica could become a popular NASCAR driver and is good enough to make a respectable showing and that might be all it takes.